Transfer-encoding chunked hex markers appearing in output?

[Henry]

I've written a very simple script which acts as a specific proxy for XML 
feeds, reading from a source URL and passing the content straight 
through. It's trivial and it's working fine when run as a standalone 
HTTP server (via node.js).

However when I put nginx in front of it I get what appear to be the 
chunk hex markers in the output - which obviously renders the XML 
invalid. Here is an example of the output, note the "f32" at line 1, and 
the repeated "1000" on lines 53 & 85, the 
"58a" on lines 125 & 137 and so on.

https://gist.github.com/1973377

Here is my (reduced) nginx conf:

   server {
       listen 80;
       server_name feeder;
       location / {
           proxy_pass http://127.0.0.1:8888;
       }
   }

I've confirmed that the chunks themselves don't contain the spurious 
data (by dumping them to a file and checking with dhex), and apart from 
the expected Server: change the HTTP headers are the same with and 
without nginx:

   HTTP/1.1 200
   Server: nginx/1.0.12
   Date: Mon, 05 Mar 2012 02:02:38 GMT
   Content-Type: text/xml; charset=UTF-8
   Connection: keep-alive
   etag: rM1jGkNUA9AzVj2iMQN9KWwLgcQ
   last-modified: Mon, 05 Mar 2012 02:07:53 GMT
   expires: Mon, 05 Mar 2012 02:08:03 GMT
   cache-control: private, max-age=0
   x-content-type-options: nosniff
   x-xss-protection: 1; mode=block
   transfer-encoding: chunked

I'm at a loss how to proceed and I'm hoping someone on the list 
recognises the problem.

In case it's relevant, here is the code for the script, with the lines 
that do the proxying highlighted:

https://github.com/hjst/feeder/blob/bd610bd981ab4b2184610775d3c6bf4b3f37d863/feeder.js#L45-47